Aerobic single sludge suspended growth biological wastewater treatment processes (often called activated sludge processes) are widely used to treat wastewaters that contain biodegradable organic matter, nitrogen and phosphorus. When configured with anaerobic and anoxic zones and appropriate in-process recirculation streams, these processes are capable of achieving significant nitrogen and/or phosphorus removal. Such configurations are typically referred to as biological nutrient removal processes. Conventional biological nutrient removal processes use heterotrophic bacteria for biological phosphorus removal and denitrification (conversion of nitrate to nitrogen gas), and autotrophic bacteria for nitrification (conversion of ammonia to nitrate).
Heterotrophic bacteria require biodegradable organic matter, which is often quantified as five-day biochemical oxygen demand (“BOD5”) or biodegradable chemical oxygen demand (“bCOD”). Consequently, sufficient biodegradable organic matter must be present in wastewater treated by these biological nutrient removal processes for effective removal of nitrogen and phosphorus. In general, 5 mg of BOD5 or more is needed to remove 1 mg of nitrogen, expressed as the total Kjeldahl nitrogen, or TKN, and 7 to 10 mg of volatile fatty acids (“VFAs”), such as acetic or propionic acid, must be present in the wastewater or generated in the anaerobic zone of the process to remove 1 mg of phosphorus, expressed as P. Biological nutrient removal is adversely affected for wastewaters containing insufficient quantities of biodegradable organic matter relative to the nutrients contained within the wastewater.
Some wastewaters are not only deficient in biodegradable organic matter relative to the nutrients to be removed but also contain reduced sulfur compounds, often in the form of hydrogen sulfide (H2S). Such wastewaters are often generated when municipal and industrial wastewaters are pre-treated with methanogenic anaerobic processes. Methanogenic anaerobic treatment of wastewater containing biodegradable organic matter generally requires significantly less energy input compared to the amount required for aerobic treatment of such wastewaters and, in fact, energy is produced in the form of methane-containing biogas. Biomass yields are also lower in these types of anaerobic processes, resulting in less residual sludge to be processed.
However, the effluent from a methanogenic anaerobic process contains most of the nutrients originally in the wastewater. Further, significant quantities of H2S and other reduced sulfur compounds can be generated during the process. Thus, the resulting wastewater often cannot be effectively treated biologically to remove nutrients because of the deficit of biodegradable organic matter. Moreover, biodegradable carbon in the form of compounds such as methanol, acetic acid, or a variety of other readily available biodegradable organic matter products must be purchased and added to the process to remove nutrients. This, of course, seems to be counter-productive as biodegradable carbon is first removed from the wastewater and then subsequently purchased and added back into it. Moreover, the reduced sulfur compounds generated during the anaerobic treatment process pass into the aerobic zone of the biological nutrient removal process, where these compounds exert a significant oxygen demand of 2 mg O2/mg H2S—S. These two results lead to increased cost and make the option of an anaerobic treatment followed by a conventional biological nutrient removal process unattractive. These results also make it unattractive to use conventional biological removal processes to treat naturally occurring wastewaters, such as certain industrial wastewaters, that have relatively significant concentrations of reduced sulfur compounds and relatively low concentrations of biodegradable matter.